A flat panel CRT display is a thin, flat display which presents an image on the display's viewing surface in response to electrons striking light emissive material. The electrons can be generated by mechanisms such as field emission and thermionic emission. A flat panel CRT display typically includes a faceplate structure and a backplate structure joined together by an outer wall along the periphery of the two plate structures. The resulting enclosure is usually held at a high vacuum, typically a pressure of 10.sup.-7 torr or less. To prevent collapse of the display under the high vacuum, one or more spacers are typically located between the plate structures.
FIG. 1 schematically illustrates a portion of a conventional flat panel CRT display 100. The components of display 100 include faceplate structure 120, backplate structure 130, spacer wall 140, and voltage source 150. Although only one spacer wall 140 is shown in FIG. 1, display 100 includes additional such spacer walls.
Faceplate structure 120 includes transparent electrically insulating faceplate 121 and light emitting structure 122 formed on the interior surface of faceplate 121. Light emitting structure 122 typically includes light emissive elements, such as phosphors, which define the active region of display 100. Faceplate structure 120 also includes an anode (not shown) adjoining light emitting structure 122 and connected to the positive (high voltage) side of voltage source 150. Backplate structure 130 consists of electrically insulating backplate 131 and electron emitting structure 132 located on the interior surface of backplate 131. Electron emitting structure 132 includes a plurality of sets 161-165 of electron emitting elements which are selectively excited to emit electrons.
Various voltages are applied to the portions of electron emitting structure 132 during display operation. All of these voltages are normally very low compared to the voltage that the positive side of voltage source 150 provides to the display's anode in faceplate structure 120. As an approximation relative to light emitting structure 122 and the adjoining anode, electron emitting structure 132 can be viewed as connected to the negative (low voltage) side of voltage source 150. FIG. 1 schematically illustrates this connection. With the anode being held at a high positive voltage (e.g., 5 kV) relative to electron emitting structure 132, the electrons emitted by the electron emitting elements in sets 161-165 impinge on corresponding light emissive elements in the light emitting structure 122, causing the light emissive elements to emit light visible at the exterior viewing surface of faceplate 121.
Spacer wall 140 is situated between the generally planar lower surface of light emitting structure 122 and the generally planar upper surface of electron emitting structure 132. With spacer 140 being made of material having a largely uniform resistivity, the electric potential field (sometimes termed voltage distribution) along spacer 140 is approximately the same as the potential field that would be present at the same location in free space, i.e., in the absence of spacer 140, between plate structures 120 and 130. Except for electrons that strike spacer 140, the presence of spacer 140 does not significantly affect the movement of electrons from electron emitting structure 132 to light emitting structure 122.
FIG. 2 schematically depicts a portion of another conventional flat panel CRT display 200. Except as described below, displays 100 and 200 are the same, similar elements being labeled with the same reference symbols. Baseplate structure 130 of display 200 additionally includes electron focusing system 133 consisting of focusing structures 133a-133f. One edge of spacer wall 140 contacts focusing structure 133a.The opposite edge of spacer 140 contacts light emitting structure 122.
Focusing system 133 is electrically connected to the negative side of voltage source 150. As a result, focusing system 133 asserts repulsive forces on the electrons emitted from the electron emitting elements in sets 161-165. These repulsive forces direct or focus electrons toward the appropriate light emitting elements of light emitting structure 122.
With focusing system 133, specifically focusing structure 133a, being at the same potential as electron emitting structure 132, the potential field along spacer wall 140 is different from the potential field that would be present at the same location in free space, i.e., again in the absence of spacer 140, between faceplate structure 120 and baseplate structure 130, now including focusing system 133. This can result in undesired deflection of electrons emitted from electron emitting elements close to spacer 140, e.g., the electron emitting elements in sets 161 and 162. It is desirable to arrange a spacer in a flat panel CRT display containing an electron focusing system so as to avoid undesirable electron deflection or to overcome undesired electron deflection that does occur.